The best way I know of is finding some lucky eggs (they give you an extra 50% XP) on wild Pelippers and giving them to the Pokemon you want to level up. Then put a good lvl 100 sweeper and go through the elite four. I've done it a lot, it works nicely.

This will only help you if you have XY, but if you have a buddy with a 3DS or if you have a second 3DS then just trade it over to XY and put it in the Daycare. Then, just go to the center of Lumios City and make your character move in circles non-stop (While riding a bike) and tape down the d-pad. Then, go to sleep and when you wake up, you have a high level Pokemon! now you just trade it back to ORAS and voila.

ok
luck eggs are a must have for fast exp
at the battle resort the trainer's all have high lv so it helps but you can only battle once a day
there is route 117 and 118 with alot of rematch trainer's
there the tv interviewers who can be found on route's 111/ 118 / 120 if you give them a interview every time you meat them they move on to next route
hope it helped